What your friend is asking for is a complete understanding and presentation of the cellular and synaptic process that leads to neurotoxicity.
That is a damned high requirement considering his own brain is at stake...
Only a true commitment to study will bring the understanding he desires.
But...
If a single example were to be pointed to as the best explanation of the cause of neurotoxicity it would be found in this thread:
http://www.bluelight.ru/vb/threads/598956-The-Brain-Damage-of-MDMA-(-quot-Ecstasy-quot-)
Again, understanding what is written is not a simple task.
But this describes the leading theory of neurotoxicity - dopamine metabolites make their way into SERTs, or serotonin transporters.
This causes damage to the receptor sites, especially in the more vulnerable higher cortex.
Cell membranes are destroyed, mitochondria is spilled out, and lipid peroxidation occurs.
That means the fat that comprises your brain reacts with the powerful alkaline genetic material and creates peroxide radicals - that ensure further destruction of the serotonin receptors and other neurons.
It could be argued that a small level of this is occurring with every single MDMA dose.
And perhaps this cellular chain of events is important to the experience itself.
But...
There are a number of ways to block neurotoxicity in research that take place AFTER the subjective effects of rolling have been experienced.
Such as taking an SSRI at three hours post dose, just as you would be coming down, to shut off the experience.
This closes the SERT to the destructive and nasty metabolites.
So it would seem, based on such data, that neurotoxicity is not necessarily a part of the euphoria.
Rather it is a consequence of it.
And a great contributing factor to this consequence is the level of serotonin depletion that occurs.
The less serotonin left, the greater the risk of severe dopamine invasion.
Does that make sense?
Re-dosing is a GUARANTEED way to blast away your available serotonin supply.
Even a single dose will cause a reliable drop in serotonin and its synthesis for weeks/months.
So multiple doses are a VERY bad idea, according to the central theory of neurotoxicity.
And according to PLENTY of anecdotal reports on BL.
Time and again the experienced rollers will tell the newbies that taking too much is a BAD idea.
And rolling two days in a row is a MUCH bigger mistake.
The third time I broke that rule I suffered severely - so much that it nearly killed me!
So tell your friend to simply listen to reason.
Besides, every single paper that studies neurotoxicty in rats and primates uses a VERY simple method to guarantee toxic changes in the brain - REDOSING.
Just read.